Screw fastening



Dec, 7, 1948. E. G. THEURER 2,455,885

SCREW FASTENING 4 Filed 001;. 1, 1945 INVENTOR Erwin G fkeurer Jam,

Patented Dec. 7, 1948 UNITED srArss PATENT OFFICE scnnw r iz r anmd Application October 1, 1945, Serial No. 819,606

4 Claims. (01. 85-32) The present invention is concerned with means for securely aillxing screws into structures of machinable material, such as plastic, wood, aluminum castings, magnesium castings and die castings generally, of shearing strength insufllclent to admit of threading a machine screw or a selltapping screw directly thereinto.

It is among the objects of the invention to provide a simple and inexpensive insert, which dispenses with the need for tapping the bore of the mounting tructure before application of the insert, which admits of being embedded completely within the thickness of the mounting structure without protruding parts. which in the applicacutting edges near the inner part of the insertv are mutilated triangles or trapezia 20, while those at the closed ends of slots l8 are completetriangles.

Thus the insert comprises an internally and externally threaded tube of metal with interruption thereof imposes no undue strain upon the- 16 plastic or other mounting structure, yet aflords an inherently secure mount, though no lock ring, nut or other auxiliary holding appliance is used, a mount the security of which is further enhanced by the application of the screw to be pmitioned in the insert, which screw inherently becomes securely, yet removably locked in place in the insert without the need for lock washer or other extraneous screw-locking instrumentalitles.

In the accompanying drawings in which are shown one or more of various possible embodiments of the several features of the invention,

Fig. 1 is a side elevation partly in cross-section showing one embodiment preparatory to introducing the insert into the mounting structure,

Fig. 2 is a view in longitudinal cross-section showing the insert (in this case devoid of the cylindrical pilot), in place in the mounting structure,

Fig. 3 is a view similar to Fig. 2 showing the relation of the parts with the mounting screw in place therein, and

Fig. 4 is a side elevation of a modified form of insert.

Referring now to the drawings, the insert is a screw I having an external machine screw thread H and an axial bore tapped at I! and serving for mounting the screw ii to be retained therein. While the external thread has been shown and in many applications is preferred of coarser pitch than the internal thread, it will be understood that the relationship may be reversed, or the threads may be of equal pitch.

Desirably the insert is tapered as by machining off the crowns of the threads at the entry end thereof along a conical locus as at 0-1:. If de-,

sired, the reduced end of the insert may have a tions, preferably slots, extending longitudinally from one end thereof, the sides of such interruptions or slots determining cutting edges. The diameter of the bore of the tube is sufficiently large through the entire length thereof to accommodate the machine screw to be inserted therein without exerting stress 'upon the insert when thus intro- 20 duced into the insert as such apart from its plastic short cylindrical pilot extension ll, as shown in Fig. 1, but omitted from Figs. 2, 3 and 4. Desirably, the insert is provided with a head L5 to be mount. The bore includes a thread of diameter and pitch to fit those of such machine screw and extending at least throughout the major portion of the length of the insert. The crest of the external thread of the insert which also extends throughout the major portion of the length of the insert is of uniform diameter at that part Of the insert length which is beyond the longitudinally interrupted or slotted portion and the crest of the external thread along at least, part of the interrupted portion .of the insert lies in a conical locus tapered to the interrupted extremity of the insert. The insert is applied into the smooth cylindricalbore l'l machined or pre-molded in the low shearing strength mounting structure S of plastic, wood or metal. for this purpose, a suitable driving tool T, to be described below, may be employed.

The pilot I will center the insert ill in the bore ll of the structure 8 and thethread cutting edges 20, I9 wlll'causethe insert to tap its way into the bore of the mounting structure. In this tapping operation, the thread is progressively cut as the sharp thread edges 20 of truncated contour at the entry end of the insert are progressively intro- 3 the socket 2| at the bottom of the insert mounting socket.

The tongues 22 dennedbetween successive slots l8 at the inner portion oi the insert. will have been forced inward as shown in exaggerated form V in Fig. 2, by the reaction of the mounting struc on each insert, but a lesser or greater number is feasible and the slots may be of greater or lesser length depending on the yieldabllity or the degree of inward closing of tongues 22 that is desired. The thickness of, the stock at the inner end of the insert, that is, the relative diameter of the inside and outside threads also determines the yieldabilityor degree of'inward closing of the tongues 22.

Any of a variety of tools may be employed for introducing the insert into the mount. One such tool 1' is shown in Fig. 1. It comprises a handle 25 having on the extremity thereof a threaded stud 28, which meshes with the tapped or threaded bore i2 of the insert. Desirably, the handle end of the tool has a flat undersurface 21 of diameter no larger than the bore of the countersink l6. Accordin ly. the tool may be threaded into the insert until its fiat 21 engages the head Illustratively, six equidistant slots II are used tapping insert is shown in Fig. 4, which involves a construction generally similar in appearance to that of Fig. 1. except that the threads 80 in this instance are preferably rolled, as is also the seu" ries of self-tapping grooves 3| which extend transverselyacross the threads at high pitch at the inner part of the insert. The thread rollin operation may be performed in manner well un- IS. The insert is then guided by its pilot ll into the bore ll of the mounting structure until the head I! bottoms in the countersink It. At that time the thread of the insert will have gripped so tightly against the wall I! of the mounting structure that the mounting tool may readily be unscrewed from the insert, without loosening the latter from the mounting structure and without injury of any kind.

Preferably the tool is provided with an unthreaded inlet or pilot stud extension 28 with only .002 to .003 inch clearance with respect to the peaks of the tapped threads H, which correspondingly limits the amount of inward deflection permitted at the tongues 22 between the slots id of the insert. This feature is desirable in order to avoid too much binding of the screw to be introduced into the insert.

Upon introduction of a commercial machine screw it into the insert, as for instance, to look a plate 29 upon the mounting structure, as shown in Fig. 3, the inner end of such screw will deflect or force the inwardly deflected tongues 22 outward to substantially the position of said tongues before introduction of the insert into the mount. Not only is the screw thereby caused securely to be anchored in the insert, but the outward deflection of the insert tongue ends 22 into the mounting structure results in looking such insert more effectively in place. It will be noted that the insert is thereby maintained securely in the mountin structure and the screw securely in the insert,

without the need for any lock washers, lock nuts, lock rings or other auxiliary securing appliances.

It will be understood that the screw II may be readily removed from the insert. without injury to or loosening oi the latter, which remains permanently in place. If desired, the insert could also be removed. especially if its head he provided with a suitable illlister slot (not shown) and will leave a smooth thread in the mounting structure in which may be securely accommodated a simiderstocd by those skilled in the art.

Since the inner end of the insert of Fig. 4, unlike that of Fig. l, is continuous and not slotted or interrupted into tongues, but is weakened along longitudinal grooves the inward deflecting action and outward forcing action of the screw it is not to be compared with the action of the embodiment of Fig. l, but the construction of Fig. 4 is yet adequate for many applications, especially on structures generally that are not subject to intense or sustained vibration in use.

As many changes could be made in the above construction, and many apparently widely dlflerent embodiments of this invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. An insert suitable for self tapping into an untapped cylindrical bore in a plastic mount and suitable for accommodating therein a machine screw having a thread of uniform diameter, cross section and pitch, said insert comprising an internally and externally threaded tube of metal having interruptions extending longitudinally from one end thereof, the sides of said interruptions determining cutting edges, the inside diameter of said tube throughout the entire length thereof being sufllciently large to accommodate such machine screw and said bore being threaded at least throughout the major portion thereof with a thread of diameter and pitch to fit that of such machine screw, said threaded portionextending from within the uninterrupted portion into the interrupted portion of said tube, the external thread of said tube being a machine thread of uniform root diameter extending along the major portion of the length of the insert, the crest of said external thread being of uniform diameter at that portion of the insert length which is beyond said interruptions, and the crest of said exterior thread along a portion of the length thereof which has the interruptions being in a conical locus taperlng to the interrupted extremity of the insert, whereby upon threading the insert into such untapped bore the cutting edges at its entry and will form a corresponding thread in the plastic and the resistance of the mount will force said interrupted portion of the tube slightly inward, so that upon the subsequent introduction of a machine screw that ilts into the insert said interrupted portion of the tube will be forced outward thereby to substantially its original form for en- 5 hanced griporthe insert in such mount and of such screw in the insert. 1

2. The combination of a plastic mount with the insert as claimed in claim 1 threaded in place therein, a portion of the insert length which has the interruptions being maintained inward slight- 1y by the encompassing mount structure.

3. A- mount of plastic having threaded in place therein the insert claimed in claim 1 and having threaded in place within said insert and along the length thereoi; a. machine screw, the said screw retaining the interrupted portion of the insert in substantially the original condition of said insert prior to the deformation incurred in introducin the same into the bore in said mount.

4. The combination recited in claim 1 in which the longitudinal interruptions are slots through the insert and the external machine screw thread is truncated along said conical locus.

" ERICK G. TI-IEURER.

assumes 6 tumultuous crrnn The ,foilowingreferences are of record in the file of this patent:

Great Britain May 20, 1937 

